Project Summary/Abstract The immune system surveys the skin and other barrier tissues such as lung and gut, but mechanisms for failed immunity in the periphery that lead to cancer or autoimmune diseases are unknown. The dendritic cell (DC) is a specialized immune sentinel that directs T cells to tolerance or immunity. DC comprise a rich network in skin consisting of several different populations, but are understudied, leaving a major gap in therapeutic intervention. We have identified distinct properties for DC in skin that relay information by migrating out to the draining lymph node (LN). We find during inflammation skin migratory DC may direct T cells to tolerance by unknown mechanisms, counter-regulating and controlling immunity. These cells are uniquely genetically programed in humans and in mouse to dampen immunity with a high expression of shared tolerance genes, including some genes such as PD-L1 that have been successfully targeted in the clinic for peripheral tissue cancers including melanoma skin cancer, colorectal cancer and lung cancer. We hypothesize homeostatic programming of skin DC leads to failed immune priming leading to impaired early detection of skin cancers. We find these pathways are of great importance as therapeutic targets to promote self-tolerance (such as during autoimmune disease) or to block tolerance (to improve immunity during cancer). Because these mechanisms are distinct they can be combined with current modalities for patients in immunotherapy of skin cancer, vaccine science, and inflammatory skin disease. In 2 discrete aims, this application will address programmatic conditioning of skin DCs and the unique cellular mechanisms by which they promote immune tolerance in both mice and humans, examining consequences during immune priming and early skin cancer growth.